The long-term objectives of this project are to determine the role of MDM2 oncogene in p21 regulation, to evaluate the potential of MDM2 and p21as novel targets for cancer chemoprevention and chemotherapy, and to translate these findings into more rational prevention and treatment of human cancers. The expression of p21 is under the control of p53. However, p21 is also regulated by p53-independent pathways. The MDM2 oncogene is overexpressed in many human cancers, with the MDM2 levels being associated with tumor progression and poor prognosis. MDM2 has a role in the regulation of p53 stability and activity. In addition, MDM2 also has p53-independent activity, which may be associated with its carcinogenic properties. Preliminary studies have indicated that MDM2 has an important role in regulation of p21. The inhibition of MDM2 with anti-MDM2 antisense oligonucleotide or siRNA targeting MDM2 significantly elevates p21 protein levels in cancer cells (p53 null). In contrast, overexpression of MDM2 diminishes the p21 level, shortening the p21 half-life. MDM2 facilitates p21 degradation independent of ubiquitination and the E3 ligase function of MDM2. Instead, MDM2 promotes p21 degradation by facilitating binding of p21 with the proteasomal C8-subunit. MDM2 functions as a negative regulator of p21, an effect independent of both p53 and ubiquitination. This application seeks to further clarify the role and mechanisms of MDM2 oncogene as a negative regulator of p21, including three Specific Aims: 1). To determine the mechanisms by which MDM2 regulates p21 at transcriptional level;2) To determine the mechanisms by which MDM2 regulates p21 at post-translational level;and 3) To determine the in vitro and in vivo activity of MDM2 and p21 human cancer growth and progression. These studies will generate knowledge on the mechanisms responsible for the p53-independent carcinogenic activities of MDM2 and evaluate the potential of these pathways for human cancer prevention and treatment.